Devices have been developed to determine and analyze the composition of microscopic particles, such as interplanetary micrometeoroids, cometary dust, galactic dust, solar dust, particles orbiting a planet (such as the particles in the rings of Saturn), droplets in clouds of planetary atmospheres, and earth orbiting debris from any one of (1) nuclear weapon explosions, (2) rocket exhaust, (3) explosions of spacecraft, (4) rockets, (5) volcano eruptions, as well as ejecta from hypervelocity impacts of missiles or jet engine exhaust. The composition of these particles has been determined by utilizing mass spectrometry apparatus for analyzing the abundancy of elements and isotopes of the elements in the particles. Such analyzers are disclosed in my U.S. Pat. No. 3,715,590, as well as in the article entitled "Detection Technique For Micrometeoroids Using Impact Ionization," written by Siegfried Auer and Kurt Sitte, which appeared in Earth and Planetary Science Letters, 1968, Volume 4, Pages 178-183.
In the prior art devices, a microscopic particle impacts on a front face of an electrically biased metallic surface, which may be tungsten. In response to the impact, positive ions are derived from the front face and directed to an ion detector by an electrostatic field established by a particle and ion pervious grid electrode that is negatively biased relative to the impact surface. The kinetic energy of an impacting particle on the metal surface results in a portion of the particle being vaporized and ionized. Since only the kinetic energy of an impacting particle causes vaporization and ionization, the prior art device has a relatively low efficiency, particularly for relatively slow impact velocities (less than five kilometers per second). The conversion of particle material to vapor and ions is considered to be of relatively low efficiency because only a small fraction (considerably less than one percent and approximately 0.001 percent) of the atoms in an impacting particle are ionized.
A further deficiency in the prior art detector is that solid fragments of the particle may impact on metal parts, other than the impact surface, that are located in a housing for the impact surface. In response to an impact on metal parts other than the impact surface, fragmentary particles are produced that have a tendency to reach the impact surface slightly after the impact of the main part of the particle; thereby, ions are derived from the impact surface at slightly displaced time intervals. Because of the different travel times of ions of different elements from the impact surface to an ion detector, it is difficult, and frequently impossible, to distinguish between ions derived from the main part of the particle and from fragments, with a resultant confusion in analysis of the particle composition. Thus, a phenomenon known as 'ghost " is frequently a problem with the prior art devices.
A further deficiency in the prior art device is that the elements in the particle which can be ionized most readily are over-represented in a mass spectrum derived from the ions. In particular, alkaline metals are always over-represented, except for particles having very high impact velocities (in excess of approximately twenty kilometers per second). Because alkalines are always over-represented, large corrections and therefore uncertainties, are necessary, based on considerations of plasma equilibrium conditions, to determine the correct proportions of the elements in the impacting particles. Alkaline elements are always over-represented with the prior art device because the particles almost invariably have some alkalines therein. The alkalines have a lower work function than other elements in the particle and are, thereby, more easily ionized. Once one alkaline ion is generated, it has a tendency to ionize additional alkaline atoms in the particle, with a resulting regenerative effect.